In microstructure manufacturing processes such as processes of manufacturing microelectromechanical system (MEMS) and through-silicon via (TSV), the necessity of a deep etching technique is increasing for forming a trench or via hole with a depth of a few μm to a few ten μm in a substrate made of a semiconductor such as Si.
As semiconductor etching, generally used are isotropic wet etching using an etching solution containing hydrofluoric acid, nitric acid, and acetic acid, anisotropic etching using an alkali solution such as tetramethylammonium hydroxide (TMAH) or KOH, and dry etching using a plasma of an etching gas such as SF6 or CF4.
In isotropic wet etching, however, etching in the widthwise direction, i.e., side etching occurs in addition to etching in the depth direction. This makes it impossible to achieve a high aspect ratio (the ratio of the depth to the width of a recess).
In anisotropic etching, high-aspect-ratio deep etching can be performed by using an etching rate difference corresponding to the crystal orientation of a semiconductor. In this case, however, the shape of a pattern to be formed by etching and the crystal orientation of a semiconductor must be so determined that the etching rate in the depth direction is much higher than that in the widthwise direction. That is, the shape of a structure to be formed by etching is limited, so design limitations are large.
The deep etching techniques using dry etching include a technique called a Bosch process. In this Bosch process, plasma etching using SF6 gas or the like and sidewall protective film formation using C4F8 gas or the like are alternately repeated. This technique has a problem that a fluorocarbon-based deposit remains on sidewalls and exerts influence on the performance of the device. This technique also has a problem that sidewalls are formed into a scalloped shape.
Recently, a method called a metal-assisted chemical etching (MacEtch) method has been developed. In this MacEtch method, a catalyst pattern made of a noble metal is formed on the surface of a semiconductor substrate, and the semiconductor substrate is dipped in a solution mixture of hydrofluoric acid and an oxidizer. That portion of the semiconductor substrate, which is in contact with the catalyst, is preferentially etched, and the catalyst moves downward as the etching progresses. Consequently, a trench or via hole whose depth direction is perpendicular to the surface of the substrate is obtained. However, this perpendicularity is not obtained depending on the conditions.